Current ophthalmic refractive surgical methods, such as cataract surgery, intra-corneal inlays, laser-assisted in situ keratomileusis (LASIK), and photorefractive keratectomy (PRK), rely on ocular biometry data to prescribe the best refractive correction. Historically, ophthalmic surgical procedures used ultrasonic biometry instruments to image portions of the eye. In some cases, these biometric instruments generated a so-called A-scan of the eye: an acoustic echo signal from all interfaces along an imaging axis that was typically aligned with an optical axis of the eye: either parallel with it, or making only a small angle. Other instruments generated a so-called B-scan, essentially assembling a collection of A-scans, taken successively as a head or tip of the biometry instrument was scanned along a scanning line. This scanning line was typically lateral to the optical axis of the eye. These ultrasonic A- or B-scans were then used to measure and determine biometry data, such as an ocular axial length, an anterior depth of the eye, or the radii of corneal curvature.
In some surgical procedures, a second, separate keratometer was used to measure refractive properties and data of the cornea. The ultrasonic measurements and the refractive data were then combined in a semi-empirical formula to calculate the characteristics of the optimal intra-ocular lens (IOL) to be prescribed and inserted during the subsequent cataract surgery.
More recently, ultrasonic biometry devices have been rapidly giving way to optical imaging and biometry instruments that are built on the principle of Optical Coherence Tomography (OCT). OCT is a technique that enables micron-scale, high-resolution, cross-sectional imaging of the human retina, cornea, or cataract. Optical waves are reflected from an object or sample and a computer produces images of cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection.
OCT may be performed based on time-domain processing of Fourier-domain processing. The latter approach includes a technique known as swept-source OCT, where the spectral components of the optical signal used to illuminate the sample are encoded in time. In other words, the optical source is swept (or stepped) across an optical bandwidth, with the interference signal produced by the combination of the source signal and the reflected signal being sampled at several points across this optical bandwidth. The sampling clock, which is typically designed to sample the interference signal at equally spaced points across the optical bandwidth, is referred to as a “k-clock,” and the resulting samples, which are samples in the optical frequency domain or “k-space,” are referred to as “k-space” samples.
In practice, the optical source is successively directed to each of a series of points on the surface of the object (e.g., the eye) being imaged, with k-space samples across the spectral bandwidth being collected at each of these points. The k-space samples corresponding to each point are processed, using well-known digital signal processing techniques, to provide image data corresponding to a range of depths in the imaged object, i.e., an “A-scan.” The A-scans across the series of points are compiled to create a B-scan; multiple B-scans, corresponding to sequential “rows” along the imaged object can be compiled to form three-dimensional image data. It will be appreciated that because of the Fourier-domain processing used in swept-source OCT, z-axis scanning, where the length of the reference arm of the interference is successively changed to obtain information at different depths in the imaged object, is not needed. Rather, depth information is obtained from the processing of the k-space samples, over a range of depths that corresponds inversely to the size of the spectral frequency increments for the k-space samples.
OCT technology is now commonly used in clinical practice, with such OCT instruments are now used in 80-90% of all IOL prescription cases. Among other reasons, their success is due to the non-contact nature of the imaging and to the higher precision than that of the ultrasound biometers.
Even with these recent advances, however, substantial further growth and development is needed for the functionalities and performance of biometric and imaging instruments.